The present invention relates to a method for manufacturing an optical fiber preform. An optical fiber is made by drawing a preform on a draw tower. A preform generally comprises a primary preform consisting of a glass tube of very high quality which forms part of the cladding and the core of the fiber. This primary preform then undergoes a overcladding or sleeving process to increase its diameter and to form a preform which can be used on a draw tower. In this context, the term inner cladding is given to the cladding formed inside the tube, and outer cladding to the cladding formed outside the tube. The homothetic drawing operation consists of placing the preform vertically in a tower and drawing a strand of fiber from one end of the preform. For this purpose, high temperature is applied locally to one end of the preform until the silica is softened, and the drawing speed and temperature are then permanently controlled during the fiber drawing operation since they determine the diameter of the fiber.
An optical fiber conventionally consists of an optical core, whose function is to transmit and optionally amplify an optical signal, and an optical cladding whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and of the outer cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber breaks down into a fundamental mode guided in the core, and into secondary modes guided over a certain distance in the core-cladding assembly and called cladding modes.
An optical fiber may be made from a preform comprising a primary preform consisting of a tube of pure or doped silica in which layers of doped and/or pure silica are successively deposited to form an inner cladding and a central core. The deposits made in the tube are of the type of Chemical Vapor Deposits, abbreviated to CVD. This type of deposit is made by injections of gaseous mixtures into the tube and ionizing said mixtures. The CVD-type deposit includes Modified Chemical Vapor Deposition (MCVD), Furnace Chemical Vapor Deposition (FCVD) and Plasma Enhanced Chemical Vapor Deposition (PCVD).
After depositing the layers corresponding to the core and inner cladding, the tube is closed on itself during a so-called collapsing operation. This produces the primary preform consisting of a silica rod. This primary preform then undergoes a overcladding process, generally with natural silica particles for cost-related reasons. For overcladding, a plasma deposit method may be used during which the natural silica particles are deposited and fused by a plasma torch under a temperature in the region of 2300° C. so that they vitrify on the periphery of the primary preform. The primary preform is caused to rotate around itself and the torch or the primary preform moves longitudinally, one with respect to the other, to ensure uniform depositing of silica around the entire periphery of the rod. The overcladding process is generally conducted in an enclosed unit under a controlled atmosphere to ensure protection against electromagnetic perturbations and the release of ozone by the plasma torch.
Overcladding by plasma deposit using silica particles is low cost, but it produces impurities which are deposited on the periphery of the primary preform. These impurities, such as water and dust particles, are derived from ambient air in the unit in which the overcladding operation is conducted. The presence of impurities in the outer cladding—formed when overcladding the primary preform—deteriorates the optical properties of the fiber, in particular when impurities are present in the first layers of silica deposited on the primary preform. This problem of impurities incorporated during overcladding becomes greater the larger the size of the central core of the primary preform. When the central core has a large diameter, the inner cladding is of limited thickness and the impurities incorporated around the periphery of the tube during the overcladding have an impact on the propagation of the signal within the central core that increases the closer they lie to the core.
Yet it is sought to manufacture preforms of large capacity. The capacity of a preform is defined as the quantity of optical fiber length which may be drawn from this preform. For preforms having the same lengths, the greater the diameter of the preform, the greater its capacity. To reduce manufacturing costs and limit connection losses, it is desirable to provide long lengths of linear fibers from one and the same preform. It is therefore sought to manufacture large diameter preforms whilst complying with relative size constraints between the diameter of the central core and the diameter of the optical cladding. The final preform after overcladding has the same ratios of core diameter to cladding diameter as the drawn optical fiber. To manufacture a large capacity preform it is generally chosen to increase the quantity of overclad rather than to increase the diameter of the primary preform which is costly to manufacture.
US 2002/0144521 describes a method for manufacturing a large capacity preform. This document suggests making a primary preform by depositing a large diameter central core inside a tube doped with Chlorine and Fluorine. The tube is doped with Fluorine to compensate for the increase in the refractive index generated by doping with Chlorine. The tube is doped with Chlorine to limit the migration of OH group impurities which deteriorate the optical transmission properties in the central core. The use of such a tube doped with Chlorine and Fluorine, tube diameters being equal, makes it possible to reduce the thickness of the inner cladding deposited in the tube in order to manufacture a primary preform having a central core having an enlarged diameter. This primary preform is then overcladded by plasma deposit to obtain a final preform of large diameter and hence of large capacity. The tube doped with Chlorine and Fluorine protects the central core against impurities brought by the overcladding process using natural silica particles.
However, such a method requires the use of a specific tube, more costly than a tube of pure silica. In addition, the presence of Chlorine in the tube does not prevent the formation of Si—OH bonds on the tube surface during the overcladding operations which modify the global index of the outer cladding and consequently the transmission properties of the optical fiber.
FR-A-2 760 449 describes a method for depositing silica on an optical fiber primary preform. This document proposes purifying the natural silica deposit during the overcladding operation. A supply pipe supplies a gaseous mixture containing Chlorine or Fluorine to the plasma torch to cause the removal of the alkaline or alkaline-earth elements that are contained in the silica particles in order to reduce the formation of OH groups on the primary preform.
It has been found however that it is not at the plasma torch that the impurities are incorporated in the silica overclad since the temperature, around 2300° C., is too high to promote the formation of bonds with the OH groups. The impurities are especially deposited in the silica when it has just vitrified on the surface of the tube before it cools. The addition of a gaseous mixture containing Chlorine or Fluorine at the plasma torch does not therefore sufficiently reduce the formation of impurities in the silica overclad.
FR-A-2 64 describes a method and device for depositing silica on an optical fiber primary preform. The rod of silica forming the primary preform is placed on a lathe in a sealed enclosure separated from ambient atmosphere and supplied with dried gas. The overcladding operation is conducted inside this enclosure. The air in the enclosure is successively subjected to filtration, compression and refrigeration, to purging with condensed water then final drying by adsorption. With said process it is possible, in theory, to eliminate most of the impurities likely to be incorporated in the silica overclad. However this is a complex solution and one which is costly to implement. The volume of the enclosure is at least 8 to 10 m3 and requires an air flow through the enclosure of around 3000 m3/h. To subject such a volume of air to the above-mentioned filtering and drying operations represents a very high operating cost, incompatible with the manufacturing costs of optical fibers.
There is therefore a need for a method of manufacturing an optical fiber preform with which it is possible to conduct the overcladding operation at low cost whilst limiting to a maximum the incorporation of impurities into the silica overclad.